This invention relates to air cathodes suitable for use in fuel cells or metal/air batteries, and to materials having utility in such cathodes, as well as to methods of making the same. Illustratively, the invention will be described with particular reference to cathodes for metal/air batteries.
Metal/air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. As is well known in the art, an air cathode is a typically sheetlike member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell, in which (during cell operation) atmospheric oxygen dissociates while metal of the anode oxidizes, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially hydrophobic (so that aqueous electrolyte will not seep or leak through it), and must incorporate an electrically conductive element to which the external circuitry can be connected; for instance, in present-day commercial practice, the air cathode is commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) containing a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air batteries using neutral electrolytes such as sea water or other aqueous saline solutions.
Thus, by way of more specific example, an illustrative aluminum/air cell comprises a body of aqueous saline electrolyte, a sheetlike air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. The discharge reaction for this cell may be written EQU 4A1+30.sub.2 +6H.sub.2 O=4A1(OH).sub.3.
As the reaction proceeds, copious production of the aluminum hydroxide reaction product (initially having a gel-like consistency) in the space between anode and cathode ultimately interferes with cell operation, necessitating periodic cleaning and electrolyte replacement. Recharging of the cell is effected mechanically by replacing the aluminum anode when substantial anode metal has been consumed in the cell reaction.
Metal/air batteries have an essentially infinite shelf-storage life so long as they are not activated with electrolyte, making them very suitable for standby or emergency uses. For example, an emergency lamp or lantern can be constructed with a metal/air battery such as an aluminum/air battery, and a separate container of electrolyte can be stored with the battery, or be readily available within its intended environment of use. When a need for use of an emergency light arises, a user can merely activate the metal-air battery (by immersing the electrode in the electrolyte) and be provided with useful light.
Notwithstanding these and other potential uses and advantages of metal/air batteries, their application has been limited owing to the cost and difficulty of producing satisfactory air cathodes. For instance, it is conventional in present-day practice to produce air cathode sheet material by extruding a mixture of carbon and fluorinated polymer and pressing the mixture onto a metal mesh. The resulting material is relatively expensive; moreover, it is difficult to extrude material having the high carbon content required for air cathodes. Other problems have been encountered in achieving and maintaining satisfactory cohesion of laminated (multilayer) air cathodes.